How do I overlay Textures in Blender Cycles?
Use the node tree you see here. The upper image node will point to that specific part of the image you want to overlay. You have to MASK those parts in the alpha channel  (I recommend you use .png with transparency) White=1 black=0 (non visible).

The second node it´s your regular image. The Texture coordinate nodes and mapping can share the plug for both images, but I´m doing it like this since I can sense in your post from what you write, this is what you want.

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BFCT_Schiller